The role of the external ear in temporary threshold shift (TTS) is being studied quantitatively in 20 adult human subjects with normal hearing. The experiment is investigating the relationship between the amount of acoustic gain provided by the external ear and TTS resulting from a prescribed noise exposure. Two measurement conditions are being used. For the control condition, sound pressure level of the fatiguing stimulus is monitored in an empty sound field without the subject present and remains the same for all subjects. For the experimental condition, sound levels are measured using a microphone inserted into the ear canal of each subject, and the fatiguing stimulus is adjusted so that its level remains constant at the eardrum of each subject. TTS is induced at three sound levels for both the control and experimental condition, for a total of six exposures. The premise is that levels monitored in the empty sound field will vary at the eardrums of the subjects, resulting in greater variability of TTS vs that produced when the fatiguing level is adjusted at the eardrum. Complete data sets have been obtained on two subjects, with additional incomplete data sets on four other subjects. Preliminary data analysis (linear regression) indicates a positive correlation (r2) between the amount of external ear gain and magnitude of TTS. Data collection is continuing on a regular basis.